Method for making structurally integrated film resistor assembly



Aug. 3, 1965 R. A. QUINN 3,198,718

METHOD FOR MAKING STRUCTURALLY INTEGRATED FILM RESISTOR ASSEMBLYOriginal Filed May 26. 1960 2 Sheets-Sheet 1 IIIIIIIIIIIIIIIA 5 I I 4Agent United States Patent 3,198,718 METHQD FQR MAKENG STRUCTURALLYINTE- GRATED FILM RESESTQR ASSEMBLY Ross A. Quinn, Pain Alto, Caiiifl,assignor to Lockheed Aircraft tlorporation, Enrbanir, (ialii.

Griginal application May 26, 1960, Ser. No. 31,946, new Patent No.2,994,846, dated Aug. 1, 1961. Bivided and this application Apr. 14,1961, Ser. No. 103,194

9C1airns. (Ci. 2%4-15) This invention generally relates to electronicresistor components and, more particularly, to the fabrication of astructurally integrated film resistor assembly.

This application is a divisional application of my pending applicationS.N. 31,946, filed May 26, 1960, now Patent No. 2,994,846.

With the increasing attention now being given to themicrominiaturization of electronic circuitry because of military andspace requirements, the development of highly stable and moreefiiciently constructed miniaturized electronic components andassemblies has taken on new importance. However, considerable problemshave arisen, such as the difficulty of obtaining components which remainstable up to the high temperatures of operation necessary in manymilitary and space applications. Also, although miniaturized electroniccompo nents have been fabricated in some cases, .the interconnectiontherebetween has remained a considerable problem.

The present invention is concerned primarily with resistor componentsand assemblies, and its broad object is to provide an improved methodfor making resistor components and assemblies.

A more specific object of this invention is to provide a method formaking structurally integrated film resistor components and assemblieswhich are stable at very high temperatures of operation.

Another object of this invention is to provide a method for making astructurally integrated film resistor assembly which permits moreefiicient use of a given volume.

Still another object of this invention is to provide a method for makinga structurally integrated filmresistor assembly which requires nosoldered interconnections between the individual resistor components ofthe assembly.

A further object of this invention is to provide a method forfabricating a structurally integrated film rcsistor component orassembly of components which is relatively inexpensive and lends itselfto mass production techniques.

In a typical embodiment of the invention the above objects are realizedby forming each resistor component as a high resistivity titanium oxidefilm on the inner surface of a hole provided in .a suitable substrate,the tubular film resistor components so formed being interconnected bymeans of a low resistivity titanium wiring pattern etched on oppositesides of the substrate. Also, one or more other types of electroniccomponents, such as diodes or capacitors may be contained in the emptyportions of the resistor component holes and suitably soldered to thewiring pattern in order to achieve a high component density.

The specific nature of the invention as well as other advantages, usesand objects thereof will clearly appear 3,393,713 Patented Auga3, 1965.

ICC

from the accompanying description and drawing in which:

FIGS. 1-6 illustrate various steps in the fabrication of a structurallyintegrated tubular film resistor component in a hole in a portion of asubstrate, in accordance with the invention.

H68. 2, 4, 6 and 8 are cross sectional front views of top views It, 3, 5and 7, respectively, taken along the lines indicated.

FIG. 9 is atop view of an embodiment of a struturally integrated filmresistor assembly in accordance with the invention.

FIG. 10 is a cross sectional front view of FIG. 9 taken along the lines10-10.

FIG. 11 is an equivalent electrical circuit diagram of the embodiment ofFIGS. 9 and 10.

Like numerals designate like elements throughout the figures of thedrawing.

FIGS. 1-8 illustrate typical steps for fabricating a structurallyintegrated tubular film resistor component in a hole 22 in a portion ofan insulative substrate 20. The substrate 20 may be any .of a variety ofsuitable materials such as fused silica, quartz, glass, alumnia andmagnesium oxide. Although FIGS. 1-8 illustrate the fabrication of only asingle resistor component, it is to be understood that any desirednumber of components can be simultaneously formed in the substrate 20 toprovide any desired predetermined resistor assembly.

As shown in FIGS. 1' and 2 a hole 22 is bored through the substrate 20for each resistor component to be provided, the diameter of the hole 22being chosen in accordance with the value of resistance desired, as willhereinafter become evident. A thin titanium film 25 is now coated on thesurfaces of the substrate 20, including the inner surface of each hole22 as shown in FIGS. 3 and '4. This maybe accomplished by a method suchas is disclosed in U .8. .Patent No. 2,746,888. However, I prefer to usethe sandwich method disclosed in my copending patent applications SerialNumbers 8,157 and 8,481, both filed on February 11, 1960, now Patents3,022,201 and 2,991,195, respectively. The thickness of the film 25 inthe drawings is exaggerated for illustrative purposes.

The flat faces of the titanium coated substrate are now etched usingwell known .etchants and paint resists to pro vide any desired titaniumwiring patterns thereon such as might be required for interconnectingthe resistor components in a desired manner. In FIGS. 5 and 6, thetitanium film leads 27 and 29 ,provided in contact with opposite ends ofthe titanium-coated hole 22 indicate the portions of the etched wiringzpattern corresponding to one resistor component. Between these titanium.film leads 27 and 29 appears the resistance between opposite ends ofthe tubular titanium 'film 25coated on the hole 22. Since theresistivity of titanium is quite small, the resistance between the filmleads 27 and 29 for the structure of FIGS. 5 and 6 is also quite small.

In order to provide a useable value of resistance, the tubular titaniumfilm 25 coated in the hole 22 is now converted intoa film ofhighzresistivity. A method which has been found well suited foraccomplishing this conversion is disclosed in my copending patentapplication Serial Number 8,480'filed February 11, 1960, now abandoned.The method disclosed in this 'copending patent application involvesconverting a titanium film into a high resistivity film bysimultaneously anodizing and etching the film in a bath essentiallyconsisting of an anodizing electrolyte and etching material capable ofetching the metal oxide formed on the titanium film as a result ofanodization thereof. The concentration of etching material in the bathis chosen so that the surface of the film is converted into oxide byanodization before being attacked by the etching material, the time ofsimultaneous anodizing and etching of the film in the bath determiningthe resultant resistivity thereof.

It has been discovered that this simultaneous anodizing and etchingtreatment achieves an amazingly uniform and more controlled reduction inthe film than could be obtained by any known etching process, therebymaking it possible to obtain very thin film of high resistivity andstability. An additional advantage which is also derived is that theresistivity of the film'increases not only because of the reducton inits thickness, but also, because when the film becomes very thin theanodization process will have converted a significant thickness of thefilm into a. high resistance metal oxide.

In a preferred embodiment of this simultaneous anodizing and etchingtechnique, a two-bath treatment is provided in which the first bathperforms the simultaneous anodizing and etching of the film as describedabove until an intermediate resistivity is obtained; then the finalvalue of resistivity is obtained in a true anodizing bath without anyetching material. This second bath is chosen so that the anodizingprocess pentrates to a greater depth than did the anodizing process ofthe first bath, thereby causing a greater portion of the film to beconverted into oxide to increase the film resistivity. Using thisgreater depth of anodiziing in the second bath without etching permitsgreater uniformity and more control of the final resistivity obtainedwithout further thinning of the film and, in addition, permits a higherresistivity to be obtained for a greater film thickness, since more ofthe film is con-. verted into a high resistance oxide.

The following specific example will now clearly illustrate the two-bathconversion technique for converting a metal film into one of highresistivity disclosed in the previously mentioned copending patentapplication. First, a suitable substrate, such as alumina, is coatedwith a titanium film of convenient thickness with a resistivity of theorder of 0.2 to ohms per square, and a suitable electrical lead wire isconnected thereto.

The substrate is then immersed in a first bath consisting of 1 gram ofsodium fluoride NaF in 200 milliliters of a 5% sulfuric acid H 80solution for a time of approximately ten minutes with an anodizingcurrent flow starting at 40 milliamperes per square centimeter and thendecreasing, and a voltage source adjustable up to 100 volts.

When the resistivity of the film reaches the order of 80 to 200 ohms persquare, the substrate is removed from the first bath and immersed in asecond bath consisting of a saturated sodium perborate NaBO solution.The anodizing current flow starts at 8 milliamperes per squarecentimeter and a voltage source is provided adjustable up to 250 volts.The substrate is held immersed in the second bath until the resistivityof the film increases to the desired value.

Using the two-bath procedure of the aforementioned copending applicationdescribed above, highly stable films having resistivities as high as5,000 ohms per square have been successfully produced.

Before subjecting the structure of FIGS. 5 and 6 to the simultaneousanodizing and etching treatment described above, the film leads 27 and29 are protected from the treatment with a suitable paint or epoxyresist. After the treatment, therefore, the titanium film on theinterior of the hole 22 in the structure of FIGS. 5 and 6 will beconverted to a film of high resistivity, the resulting film resistorcomponent 50 obtained being shown in FIGS. 7 and 8. The converted highresistivity film 125 is indi- 4 cated in FIG. 8 by doublecross-hatching. The unchanged low resistivity titanium film leads 27 and29 are shown in FIGS. 7 and 8 with the protective paint or epoxy resistwhich was provided during the conversion treatment removed.

FIGS. 9 and 10 are respectively top and cross-sectional front views ofan embodiment of a four resistor assembly comprising the tubular filmresistor components 50, 50, 50" and 50' which may be simultaneouslyfabricated in the substrate 20' as just described. The titanium filminterconnection pattern on the top face of the substrate 20' isindicated at 27 and on the bottom face as 29'. As in FIG. 8, the doublecross-hatched films correspond to the converted high resistivity filmswhile the single cross-hatched films 27' and 29' correspond tounconverted low resistivity titanium films.

If desired a component may be contained in any or all of the empty holesof the tubular film resistor components in order to achieve a highcomponent density, such as is illustrated by a diode 75 in the hole ofthe resistor component 50 with the diode lead wires 76 and 77respectively connected to the titanium film interconnection patterns 27and 29' as shown in FIGS. 9 and 10.

In the assembly of FIGS. 9 and 10 the resistors are shown as being allin one line. This has been done mere- 1y for illustrative convenience,and it will be realized that any other desired arrangement of resistorcomponents could be employed. Also, it will be realized that any desiredinterconnection pattern between resistor components is readily providedby etching the desired interconnection patterns 27' and 29'. FIG. 11shows the electrical circuit diagram for the particular illustrativeinterconnection patterns 27 and 29 shown in FIGS. 9 and 10.

The determination of the resistance value of each tubular film resistorcomponent in an assembly such as shown ,in FIGS. 9 and 10 will becomeevident from the following considerations.

First, as a result of the simultaneous fabrication treatment of theresistor components previously described, it will be realized that theresistivity of the high resistiviy films 125 will be the same for allholes regardless of their diameter. Thus, it can mathematically be shownthat the resistance R of any resistor component may be written as:

where p is the resistivity in ohms per square of the converted films125, L is the thickness of the substrate 20' (that is, the length of thehole) and d is the diameter of the originally bored hole 22 in FIGS.1-8. The above equation assumes that the thickness of the highresistivity film is very much smaller than the diameter d, which isusually the case.

The relative values of the resistor components 50, 50', 50" and 50", maythus be chosen by appropriately choosing their diameters d in properrelation to one another. The conversion treatment which produces theresultant film resistivity p is then employed to provide the resistivitywhich will give the desired absolute values to the resistor components.For example, if the resistor components 50, 50', 50 and 50" haveoriginal hole diameters d equal to .080, .040, .016 and .008 inch,respectively, the resistivity p is made equal to 1,000 ohms per squareand the length of L of the substrate is equal to .25 inch, then theresistor components will have substantially the following resistancevalues:

Ohms Resistor component 50 1,000 Resistor component 50 2,000 Resistorcomponent 50" 5,000

Resistor component 50" 10,000

In the embodiments and methods described herein, it will be noted thattitanium has been used as the basic material from which the resultantstructurally integrated assembly is fabricated. It is to be understood,that the invention is not limited to the use of titanium or to thespecific arrangements and techniques described herein. Other materialsand other techniques and arrangements could also be employed by means ofwhich a high resistivity film can be provided on the interior surfacesof one or more holes in a substrate with interconnection patternstherebetween.

However, the use of titanium as described is advantageous in that it isstable at very high temperatures and the conversion treatment forobtaining a high resistivity film therefrom disclosed in my copendingpatent application Serial Number 8,480 results in stable films of highresistivity. This conversion treatment may also be successfully employedwith zirconium, hafnium and uranium as well as titanium.

The above modifications and variations indicated are not exhaustive andthe invention is to be considered as including all possiblemodifications and variations in the construction, arrangement andfabrication procedure coming within the scope of the invention asdefined in the appended claims.

I claim:

1. A method of making a tubular film resistor component of astructurally integrated resistive assembly which comprises boring a holeof predetermined diameter into an insulative substrate, coating theinner surface of said hole with titanium to form a titanium filmthereon, and then converting said film to a film of high resistivity bysimultaneously anodizing and etching said film in a bath essentiallyconsisting of an anodizing electrolyte and etching solution consistingof one gram sodium fluoride NaF in 200 milliliters of a 5% sulphuricacid H 80 solution for etching the metal oxide formed on the film as aresult of anodization thereof, the concentration of etching material insaid bath being chosen so that the surface of said film is convertedinto oxide before being attacked by the etching material, the time ofsimultaneous anodizing and etching of said film in said bath determiningthe increase in resistivity thereof.

2. A method of making a structurally integrated film resistor assemblywhich comprises boring a plurality of holes of predetermined diametersin an insulative substrate, coating the surfaces of said substrateincluding the inner surfaces of said holes with a conductive metal ofthe group consisting of titanium, zirconium and hafnium to form aconductive film thereon, etching the conductive film on the faces ofsaid substrate to form a predetermined conductive film wiring patternthereon, applying a protective coating to a plurality of low resistivitythin film leads interconnecting the conductive surfaces of said holes,and then converting the conductive films on the inner surfaces of saidholes into films of predetermined high resistivity.

3. A method of making a structurally integrated film resistor assemblywhich comprises boring a plurality of holes of predetermined diametersin an insulative substrate, coating the surfaces of said substrateincluding the inner surfaces of said holes with titanium to form atitanium film thereon, etching the titanium film on the faces of saidsubstrate to form a predetermined titanium film Wiring pattern thereon,including interconnecting leads between the interior coating of saidholes, coating the titanium film wiring pattern with a protectivematerial, and then converting the titanium film on the inner surfaces ofsaid holes into a film of high resistivity by immersing the holes ofsaid substrate in a bath essentially consisting of an anodizingelectrolyte and etching solution consisting of one gram of sodiumfluoride NaF in 200 milliliters of a 5% sulphuric acid H 80 solution foretching the metal oxide formed on the film on the inner surfaces of saidholes as a result of anodization thereof, the concentration of etchingmaterial in said bath being chosen so that the film in said holes isconverted into oxide before being attacked by the etching material, thetime of immersion of said holes in said bath determining the increase inresistivity of the film on the inner surfaces of said holes, saidprotective material coated on the titanium film wiring patternprevent-ing anodizing or etching thereof during said converting.

4. A method of making a plurality of thin film resistors as componentsof a structurally integrated assembly each 'of different resistivevalues which comprises forming a plurality of apertures of predeterminedsizes and configurations in a thin insulative substrate, coating thesurfaces of said substrate including the inner surfaces of saidapertures with a conductive metal of the group consisting of titanium,zirconium and hafnium to form a conductive film thereon, etching theconductive film on the surfaces of said substrate to form apredetermined conductive film wiring pattern thereon, applying aprotective coating to a plurality of low resistance thin film leadsinterconnecting the conductive surfaces of said apertures, and thenconverting the conductive films on the inner surfaces of said aperturesinto films of predetermined high resistivity.

5. A method of making a plurality of non-planar thin film resistors on athin insulative substrate as components of a structurally integratedassembly which comprises forming a plurality of apertures ofpredetermined sizes and configurations in the substrate, coating thesurface of said substrate including the inner surfaces of said apertureswith a conductive metal of the group consisting of titanium, zirconiumand hafnium to form a conductive film thereon, etching the conductorfilm on the surfaces of said substrate to form a predeterminedconductive film wiring pattern thereon, applying a protective coating toa plurality of low resistive thin film leads interconnecting theconductive surfaces of said apertures, and then converting theconductive films on the inner surfaces of said apertures into films ofpredetermined high resistivity.

6. A method of making a non-planar film resistor component of astructurally integrated resistive assembly which comprises forming anaperture of predetermined size and configuration into an insulativesubstrate, coating the inner surfaces of said aperture with titanium toform a titanium film thereon, and then converting said film to a film ofhigh resistivity by simultaneously anodizing and etching said film in abath essentially consisting of an anodizing electrolyte and etchingsolution consisting of one gram of sodium fluoride NaF in 200milliliters of a 5% sulphuric acid H solution for etching the metaloxide formed on the film as a result of anodization thereof, theconcentration of the etching material in said bath being chosen so thatthe surface of the film is converted into oxide before being attacked bythe etching material, the time of simultaneous anodizing and etching ofsaid film in said bath determining the increase in resistivity thereof.

7. A method of forming a structurally integrated resistive asembly on athin insulative substrate and accurately controlling the resistance of aplurality of nonplanar thin film resistors during the process ofmanufacture which comprises forming a plurality of apertures ofpredetermined sizes and configurations in a substrate, coating thesurface of said substrate including the inner surfaces of saidapertures, with a conductive metal of the group consisting of titanium,zirconium and hafnium to form a conductive film thereon, etching theconductive film on the surfaces of said substrate to form apredetermined conductive film wiring pattern thereon, applying aprotective coating to a plurality of resistive thin film leadsinterconnecting the conductive surfaces of said apertures, and thenconverting the conductive film on the inner surfaces of said aperturesinto films of predetermined high resistivity through the use of a twobath treatment in which a first bath performs simultaneous anodizing and7 etching of the thin film resistors until-an intermediary resistivityis obtained, and then by a final anodizing bath without etching toobtain the desired resistances for the resistors of the assembly.

8. The invention in accordance with claim 7, wherein said first bath isfurther defined as a solution consisting of one gram of sodium fluorideNaF in 200 milliliters of a 5% sulphuric acid H 50 solution.

9. The invention in accordance with claim 8 wherein said final anodizingbath is further defined as a solution consisting of a saturated sodiumperborate NaBO solution.

References Cited by the Examiner UNITED STATES PATENTS 1,985,118 12/34Van Geel 204-56 2,443,018 6/48 Arvin et al 20155 2,920,018 1/60 Miller204-56 2,934,480 4/60 Slomin 204-37 2,943,956 7/60 Robinson 20415

2. A METHOD OF MAKING A STRUCTURALLY INTEGRATED FILM RESISTOR ASSEMBLYWHICH COMPRISES BORING A PLURALITY OF HOLES OF PREDETERMINED IAMETERS INAN INSULATIVE SUBSTRATE, COTAING THE SURFACES OF SAID SUBSTRATEINCLUDING THE INNER SURFACES OF SAID HOLES WIHT A CONDUCTIVE METAL OFTHE GROUP CONSISTING OF TITANIUM, ZIRCONIUM AND HAFNIUM TO FORM ACONDUCTIVE FILM THEREON, ETCHING THE CONDUCTIVE FILM ON THE FACES OFSAID SUBSTRATE TO FORM A PREDETERMINED CONDUCTIVE FILM WIRING PATTERNTHEREON, APPLYING A PROTECTIVE COATING TO A PLURALITY OF LOW RESISTIVITYTHIN FILM LEADS INTERCONNECTING THE CONDUCTIVE SURFACES OF SAID HOLES,AND THEN CONVERTING THE CONDUCTIVE FILMS ON THE INNER SURFACES OF SAIDHOLES INTO FILMS OF PREDETERMINED HIGH RESISTIVITY.